A conventional method of producing a silicon photovoltaic cell element is explained.
First, in order to improve efficiency by promoting an optical confinement effect, a p-type silicon substrate having a textured structure formed on its light receiving surface is prepared. Then, the p-type silicon substrate is subjected to treatment in a mixed gas atmosphere of phosphorus oxychloride (POCl3), nitrogen and oxygen at a temperature of from 800° C. to 900° C. for several ten minutes, thereby uniformly forming an n-type diffusion layer. In this method, since phosphorus diffusion is performed using a mixed gas, an n-type diffusion layer is formed not only at a light-receiving surface but also at back and side surfaces. Therefore, side etching is performed in order to remove an n-type diffusion layer formed at side surfaces. In addition, an n-type diffusion layer formed at a back surface needs to be converted into a p+-type diffusion layer. Therefore, an aluminum paste including an aluminum powder and a binder is applied to the entire back surface, and this is subjected to thermal treatment (sintering) in order to convert the n-type diffusion layer to a p+-type diffusion layer and to form an aluminum electrode, thereby obtaining an ohmic contact.
However, an aluminum electrode formed from an aluminum paste has a low electric conductivity. Therefore, an aluminum electrode, which is generally formed on an entire back surface, usually has a thickness of from about 10 μm to 20 μm after the thermal treatment (sintering) in order to reduce the sheet resistance. Furthermore, since there is a great difference between thermal expansion coefficients of silicon and aluminum, a large internal stress is generated in a silicon substrate on which an aluminum electrode is formed during thermal treatment (sintering) and cooling, thereby causing a damage to a crystalline interface, an increase in crystal defect, and warpage.
In order to solve the problems as described above, there is a method of reducing the thickness of the back surface electrode layer by reducing the amount of an aluminum paste to be applied. However, reducing the amount of aluminum to be applied results in insufficient amount of aluminum to diffuse from the surface to the inside of a p-type silicon semiconductor substrate. As a result, a desired BSF (Back Surface Field) effect (effect to enhance collection efficiency of generated carriers by the existence of a p+-type diffusion layer) cannot be achieved and the properties of a photovoltaic cell are deteriorated.
With reference to the above, a point contact method, in which an aluminum paste is applied onto a part of a silicon substrate surface to locally form a p+-type diffusion layer and an aluminum electrode (see, for example, Japanese Patent No. 3107287) is proposed.
In a case of a photovoltaic cell having a point contact structure at a surface opposite to the light-receiving surface (hereinafter, also referred to as a “back surface”), it is necessary to suppress a recombination velocity of minority carriers at a surface other than a region at which the aluminum electrode is formed. As a passivation layer for a back surface used for this purpose, a SiO2 film is suggested (see, for example, Japanese Patent Application Laid-Open (JP-A No. 2004-6565). As a passivation effect achieved by forming a SiO2 film, there is an effect of reducing the surface level density, which causes recombination, by terminating a dangling bond of a silicon atom in a back surface portion of a silicon substrate.
As another method to inhibit recombination of minority carriers, there is a method of reducing a minority carrier density by means of an electric field that generates a fixed charge in the passivation layer. Such a passivation effect is generally referred to as an electrical field effect, and an aluminum oxide (Al2O3) layer and the like are suggested as a material having a negative fixed charge (see, for example, Japanese Patent No. 4767110).
Such a passivation layer is generally formed by a method such as an ALD (Atomic Layer Deposition) method, a CVD (Chemical Vapor Deposition) method and the like (see, for example, Journal of Applied Physics, 104 (2008), 113703-1 to 113703-7). As a simple method of forming an aluminum oxide film on a semiconductor substrate, a method employing a sol gel process is suggested (see, for example, Thin Solid Films, 517 (2009), 6327-6330, and Chinese Physics Letters, 26 (2009), 088102-1 to 088102-4).